Transitions are words, phrases, and sentences writers use to help readers move easily from one sentence to another or from one paragraph to another.
Writers use transitions to tell readers, "This is the direction my train of thought is going to take next."
Here are examples of three different transitions (indicated by bold face).
Each one gives the reader a different signal. The first says, "Get ready for a change of direction"; the second says "Here's an example of what I just said." The third transition tells readers to expect some new information that will add to the previous point.

For most of us, the word "pirates" summons up Johnny Depp playing Captain Jack Sparrow in one of the three Pirates of the Caribbean films.
Real pirates, however, have none of Depp's bizarre and appealing charm.
The pirates of today haunt the coasts of Africa and Asia, armed with machine guns, machetes, and hand grenades; when they highjack a ship, they don't think twice about killing the crew or its passengers.

Believe it or not, animals can be heroic.
For instance, if a group of apes are under threat, the leader will turn and face the attackers so that the remaining group members can flee.

Among sleep researchers, lucid dreaming is not taken very seriously.
Lucid dreamers claim to be aware of when they are dreaming but have no way to prove their claims.
Moreover, only a very few people claim to have had this experience, making the claim not especially worthy of serious scrutiny.

Particularly if you are struggling with a difficult passage, pay attention to the beginnings of sentences, where transitional words and phrases are most likely to appear.
Attending to transitional words and phrases at the beginning of sentences will help you stay in tune with the writer and help you avoid going off in a direction the writer never intended.
For the same reason, it pays to be on the lookout for short transitional sentences like the one that follows:

Until quite recently, law suits claiming discrimination against workers with family obligations were rare.
But that situation has changed dramatically in the last few years.
One reason for the change is the 2000 publication of a book called Upending Gender by Joan C. Williams.
Williams argued that the workplace is organized around an old-fashioned, nineteen-fifties notion of a male breadwinner whose wife could stay home and take care of the family.
She also suggested that law suits by unhappy employees might be a last-resort option if employers would not recognize the need for change.
Many employees have taken the hint and sued their employers for discrimination based on an employee's need to provide family care duties.

Sentences like the bold one above reverse the introductory train of thought and pave the way for the real point of a passage.
They are definitely worthy of your attention, as are all transitional devices an author uses.

Click here for a comprehensive list of transitional words and phrases.